Len McCluskey won the election in April with a margin of 80,000 more votes than his opponent Jerry Hicks. Photograph: Alamy

A rival to Len McCluskey in this year's Unite leadership election has lodged a complaint calling for the result to be annulled after it emerged that 156,000 ballot papers were sent to members no longer paying fees.

McCluskey, 63, won the election in April with a margin of 80,000 more votes than his opponent Jerry Hicks, an unemployed former Unite left-wing shop steward, to win another five-year term as general secretary.

But Hicks is complaining that between the end of December last year and January this year the union increased the number of potential voters in the leadership race by about 150,000.

The complaint has emerged as trade unionists gathered for their annual conference in Bournemouth. Hicks has written to the trade union watchdog – the certification officer - to ask how "former trade union members" could be allowed to vote in the leadership election. The certification officer has wide-ranging powers and in 2011 forced construction union UCATT to re-run a leadership election, declaring the vote invalid.

Hicks says that he was told by Electoral Reform Services, which oversaw the union ballot, that Unite had identified "there was a group of 'members' who the Union had considered no longer to be members of the union" but "it was decided that they should be treated as continuing members and therefore for the purpose of the election eligible to vote".

Hicks said he wanted "Unite's leadership to explain why people who weren't members of the Unite union could vote in the election of its general secretary. So far I've been offered no explanation."

Hicks, who was once active in far-left politics, said McCluskey called the "snap election" only after the union's executive council last September changed the rule book to abolish the retirement age of the general secretary and proposed doubling the number of branch nominations required to stand in a leadership election from 50 to 100.

"They thought that would deter me but I am an ordinary member who managed to get more than 150 nominations," said Hicks. "The leadership got arrogant about their own power. They are paid six figure sums and have lost touch with ordinary people."

Jody Atkinson, Hicks' barrister, said that this client had to "act within six months of the election. As we had no substantial reply to our questions from Unite the complaint had to be issued by October".

Atkinson, a well-known expert in trade union law, said it "appeared that Unite had been balloting people who had left the union, most likely because they had not paid their subscriptions. However our investigations show that the union did not lose 156,000 members between 2010 and 2012 and it seems that ballot papers have been sent to people who have not been members for years."

McCluskey, who has been Unite's general secretary since 2010, had called the election early arguing he did not want to take to the hustings in the run up to the general election in 2015. Labour's biggest donor by far is Unite, which has provided 20%, about £12m, of party donations since the election.

Unite said it would "cooperate fully with any inquiries the certification officer may choose to make on the basis of [Hicks'] complaint".

A spokesman added that the 156,000 "members were in arrears and under rule entitled to vote in the general secretary election. Because they are in arrears they are not included in Unite's declared membership figures".

"Unite is fully confident that this was a proper decision, that the election was conducted with integrity and in full conformity with the law, and that the outcome cannot be called into question".

• This article was amended on 17 September 2013 to clarify that Unite's executive council abolished the retirement age of the general secretary, rather than raising it, and that it did not double the number of branch nominations required to stand in a leadership election, but proposed doing so. The proposal was defeated.